Methods: 1) Seven different reconstruction methods are tested on the same data. These methods include both direct reconstructions of the NH mean temperature and field reconstruction methods. The field methods include both the original method of Mann et al. 1998, the RegEM Ridge and RegEM TTLS methods used in more recent work.

2) We use a field surrogate method to estimate the stochasticity that is always present in regression methods.

Results: 1) All methods strongly underestimates the amplitude of low-frequency variability and trends. This means that it is almost impossible to conclude from reconstruction studies that the present period is warmer than any period in the reconstructed period.

2) There is a large element of chance in the reconstructions. This might also explain some of the opposing results obtained in previous studies.

These methods are closely related to the temperature reconstruction methods.Now tide-gauge measurements take the place of temperature proxies,and satellite altimetry the place of temperature observations.For the sea level reconstructions the situation is simpler than fortemperature reconstructions because the tide gauges are direct measurementsof sea level and do not include noise. We can therefore easieridentify the sources of the errors in the reconstructionsand stratify the errors into contributions from limited spatial coverageof gauges and non-stationarity of the sea level field.